The present invention relates to a microelectromechanical device and method for making the device. In particular, the invention relates to a microelectromechanical device having a support substrate that includes mesas for supporting the doped region of a partially sacrificial substrate which defines various mechanical and/or electro-mechanical members of the microelectromechanical device and a method for making the device.
Traditionally, the miniaturization of mechanical and/or electromechanical systems has been frustrated by limitations on the manufacture of small lightweight mechanical or electromechanical parts. The intricacy of the parts made their manufacture on a small scale difficult and impractical. For instance, until recently, heavy and large gimbal systems were used for navigational guidance systems in the aerospace industry. These systems contained mechanical parts that were formed of metal and were generally large and heavy. However, the intricacy of the mechanical parts made miniaturization of the navigational guidance system difficult.
However, in recent years with the proliferation and increased precision in semiconductor fabrication procedures, many of the mechanical and electromechanical structures in a mechanical system may now be replaced by microelectromechanical structures (MEMS) that are fabricated by semiconductor fabrication techniques. For instance, some gimbal systems have been replaced by gyroscopes that include one or more MEMS devices. An example of these gyroscopes is described in U.S. Pat. No. 5,650,568 to Greiff et al., the contents of which are incorporated herein by reference.
The Greiff et al. ""568 patent describes a gimballed vibrating wheel gyroscope for detecting rotational rates in inertial space. The typical gimbals of the traditional larger and heavier mechanical systems have been replaced by the lightweight, miniaturized MEMS devices. These MEMS devices that form the various mechanical and/or electromechanical parts of the gyroscope are fabricated with conventional semiconductor techniques. The electrical properties of the gyroscope are then used to provide power to these parts and to receive signals from the parts.
An important advantage in the use of MEMS devices for mechanical and electromechanical systems is the reduction of size and weight that can be achieved over the conventional mechanical systems that use metal parts. However, many mechanical and electromechanical systems, such as the gimballed systems described above, have many moving parts that must be accurately fabricated in order to operate properly with the requisite accuracy and precision. Thus, the ability to replace metallic parts with MEMS devices fabricated by semiconductor techniques is limited by the precision that can be achieved with the semiconductor fabrication techniques. Although current semiconductor fabrication techniques have been utilized to manufacture MEMS devices, these fabrication procedures still present several limitations as described below in conjunction with the Greiff et al. ""568 patent.
In this regard, FIGS. 1A-1D illustrate a conventional method for manufacturing MEMS devices with conventional semiconductor fabrication techniques. The process illustrated in these figures is commonly known as a Dissolved Wafer Process (DWP) and is described in the Greiff et al. ""568 patent.
In particular, with reference to FIG. 1A, a silicon substrate 10 and a support substrate 12 are shown. In a typical MEMS device, the silicon substrate is etched to form the mechanical and/or electromechanical members of the device. The mechanical and/or electromechanical members are generally supported above the support substrate such that the mechanical and/or electromechanical members have freedom of movement. This support substrate is typically made of an insulating material, such as Pyrex(copyright) glass.
As illustrated in FIG. 1A, support members 14 are initially etched from the inner surface of the silicon substrate. These support members are commonly known as mesas and are formed by etching, such as with potassium hydroxide (KOH), those portions of the inner surface of the silicon substrate that are exposed through an appropriately patterned layer of photoresist 16. Preferably, the etching is continued until mesas 14 of a sufficient height have been formed.
With reference to FIG. 1B, the etched inner surface 18 of the silicon substrate is thereafter doped, such as with boron, to provide a doped region 20 of a predetermined depth such that the silicon substrate 10 has both a doped region 20 and an undoped sacrificial region 22. Referring to FIG. 1C, trenches are then formed, such as by a reactive ion etching (RIE), that extend through the doped region 20 of the silicon substrate 10. These trenches form the mechanical and/or electromechanical members of the MEMS device.
As shown in FIGS. 1A-1C, the support substrate 12 is also initially etched and metal electrodes 26 and conductive traces (not shown), are formed on the inner surface of the support substrate. These electrodes and conductive traces will subsequently provide electrical connections to the various mechanical and/or electromechanical members of the MEMS device.
Once the support substrate 12 is processed to form the electrodes and conductive traces, the silicon substrate 10 and the support substrate 12 are bonded together. With reference to FIG. 1D, the silicon and support substrates are bonded together at contact surfaces 28 on the mesas 14, such as by an anodic bond. As a final step, the undoped sacrificial region 22 of the silicon substrate is etched away such that only the doped region that comprises the mechanical and/or electromechanical member of the resulting MEMS device remains. The mesas that extend outwardly from the silicon substrate therefore support the mechanical and/or electromechanical members above the support substrate such that the members have freedom of movement. Further, the electrodes formed by the support substrate provide an electrical connection to the mechanical and/or electromechanical members through the contact of the mesas with the electrodes.
As known to those skilled in the art, the step of reactive ion etching trenches through the doped region 20 of the silicon substrate 10 must be controlled to insure that the resulting trenches are accurately located on the inner surface of the partially sacrificial substrate and that the widths and walls of the trenches are formed with great precision. For example, the trenches formed by RIE must generally be formed to within a tolerance of 0.1 to 0.2 microns of a predetermined width. Further, it is important that the trenches extend completely through the doped region. As such, the inner surface of the silicon substrate must be planar and the doped region must have a predetermined thickness. While the silicon substrate 10 can be initially formed to have a planar inner surface, the inner surface of the silicon substrate is thereafter etched to form the mesas 14. Unfortunately, an etching process will produce a surface that is no longer planar, but which, instead, has a number of surface irregularities. Accordingly, because the surface is non-planar, the subsequent RIE step cannot produce trenches that are accurately located on the inner surface doped region of the partially sacrificial substrate nor can the walls of the trenches be etched with great precision. Further, the subsequent RIE step may not produce trenches that extend completely through the doped region or may produce trenches that extend too far into the undoped sacrificial region of the silicon substrate.
Thus, a method is needed for manufacturing MEMS devices utilizing semiconductor fabrication techniques that separate the various mechanical and/or electromechanical members by means of RIE through the planar inner surface of a silicon substrate such that the trenches formed by RIE extend completely through the doped region of the sacrificial silicon substrate and are precisely located on the inner surface of the doped region of the silicon substrate and have precisely defined walls. Further, a method is needed for manufacturing a gimballed vibrating wheel gyroscope for detecting rotational rates in inertial space, where the mechanical and/or electromechanical parts of the gyroscope are manufactured with increased precision.
As set forth below, the method for forming a MEMS device and the associated MEMS device of the present invention overcome the deficiencies identified with conventional methods. In particular, the method of the present invention separates the various mechanical and/or electromechanical members of the MEMS device by etching, such as by RIE, through the inner surface of a partially sacrificial substrate that is planar such that the resulting trenches are precisely defined in terms of dimension, position and depth. In particular, for a MEMS device constructed primarily from the doped region of a partially sacrificial substrate and a support substrate, the method of the present invention etches the support mesas from the support substrate instead of the partially sacrificial substrate. By etching the mesas from the support substrate rather than the inner surface of the partially sacrificial substrate, the inner surface of the partially sacrificial substrate remains planar for the etching procedures that separate the precise mechanical and/or electromechanical members. As such, MEMS devices having mechanical and/or electromechanical members can be reliably produced.
Further, the present invention provides a MEMS device that includes a support substrate having mesas that extend outwardly therefrom. The MEMS device of the present invention also includes a partially sacrificial substrate from which the mechanical and/or electromechanical members are formed that is supported by the mesas. Because the mesas are formed from the support substrate as opposed to the partially sacrificial substrate, the inner surface of the partially sacrificial substrate remains planar for later etching to separate the mechanical and/or electromechanical members.
These and other advantages are provided, according to the present invention, by a method for forming a MEMS device that initially provides a partially sacrificial substrate having a planar inner surface. The partially sacrificial substrate is doped such that the partially sacrificial substrate includes both a doped region and an undoped sacrificial region, wherein the doped region is adjacent to the inner surface of the partially sacrificial substrate. The method of the present invention also provides a support substrate, typically formed of a dielectric material, for supporting the partially sacrificial substrate. To suspend the partially sacrificial substrate above the support substrate the method of the present invention includes the step of forming at least one mesa on the support substrate, such that the mesa extends outwardly from the remainder of the inner surface of the support substrate.
The method of the present invention further includes the step of bonding the inner surface of the partially sacrificial substrate to the mesa such that the partially sacrificial substrate is suspended above the remainder of the support substrate and the doped region of the partially sacrificial substrate is in a facing relationship with the support substrate. By forming the mesas on the support substrates, the inner surface of the partially sacrificial substrate remains planar to facilitate forming the mechanical and/or electromechanical members of the MEMS device.
The method of the present invention preferably further includes the step of dissolving or otherwise removing the undoped sacrificial region of the partially sacrificial substrate after the partially sacrificial substrate has been bonded to the mesas of the support substrate. By removing the undoped region of the partially sacrificial substrate, the mechanical and/or electromechanical members formed from the doped region of the partially sacrificial substrate have freedom to move. Further, the removal of the undoped sacrificial region of the partially sacrificial substrate reduces the retention of heat by the resulting MEMS device.
Another important aspect of the present invention is the establishment of appropriate electrical connections with the mechanical and/or electromechanical members of the MEMS device. To facilitate these connections, the method of the present invention includes the step of depositing a conductive material on at least a portion of the surface of the mesa to thereby form an electrode that is in electrical communication with the doped region of the partially sacrificial substrate once the partially sacrificial substrate and the support substrate have been bonded. The electrodes can then establish an electrical connection with the mechanical and/or electromechanical members of the MEMS device.
According to one advantageous embodiment, the method of the present invention includes the step of forming at least one mesa that has a contact surface extending between a set of sloped sidewalls. As such, the conductive material can be deposited on at least a portion of the contact surface and at least one sloped sidewall of the mesa. Because the sidewalls of the mesa are sloped, the metallic material can be more readily deposited on the mesa.
In a further embodiment of the method of the present invention, the mesa having sloped sidewalls is formed by applying a photoresist layer on the surface of the support substrate at locations corresponding to the predetermined locations of the mesas. The photoresist layer is preferably dimensionally sized to approximate the dimensions of the respective bases of the mesas. The method of this embodiment further includes the step of etching the exposed surface of the support substrate while simultaneously gradually reducing the dimensional size of the photoresist layer such that the resulting mesa has a contact surface extending between a set of sloped sidewalls. Since the photoresist layer shrinks during the etching process, the contact surface of each mesa is smaller than the corresponding base.
The present invention also provides a MEMS device having precisely defined mechanical and/or electromechanical members. The MEMS device of this embodiment includes a semiconductive substrate that has an inner surface. The MEMS device of the present invention further includes a support substrate, typically formed of dielectric material, having at least one outwardly extending mesa for supporting the semiconductive substrate. Each mesa includes a contact surface that supports the semiconductive substrate such that the semiconductive substrate is suspended above the remainder of the support substrate. Because the mesas extend outwardly from the support substrate and are thus, not etched from on the semiconductive substrate, the MEMS device can include mechanical and/or electromechanical members that are precisely defined.
The mesas of the MEMS device can include a contact surface that extends between two sloped sidewalls. In addition, the MEMS device of this embodiment can also include an electrode that is formed on both the contact surface and at least one sloped sidewall of the mesa since the sloped sidewalls allow the metallic material which forms the electrodes and traces to be more easily deposited.
Accordingly, the fabrication method and associated MEMS device of the present invention provide mesas that are etched from the support substrate. By etching the mesas from the support substrate, the inner surface of the partially sacrificial substrate remains planar such that the mechanical and/or electromechanical members of the resulting MEMS device may be formed from the partially sacrificial substrate with greater precision and reliability.